The mechanical processing of the semiconductor wafer serves for removing sawing undulations, removing the surface layers that were damaged in crystalline fashion by the rougher sawing processes or were contaminated by the sawing wire, and primarily for global leveling of the semiconductor wafers. Surface grinding (single-side, double-side) and lapping are known here, and also mechanical edge processing steps.
In the case of single-side grinding, the semiconductor wafer is held on the rear side on a support (“chuck”), and leveled on the front side by a cup grinding disk with rotation of support and grinding disk and slow radial delivery. Methods and apparatuses for the surface grinding of a semiconductor wafer are known from U.S. Pat. No. 3,905,162 and U.S. Pat. No. 5,400,548 or EP-0955126, for example. In this case, a semiconductor wafer is fixedly held by one surface thereof on a wafer holder, while its opposite surface is processed by means of a grinding disk by virtue of the fact that wafer holder and grinding disk rotate and are pressed against one another. In this case, the semiconductor wafer is fixed on the wafer holder in such a way that its center substantially corresponds to the rotation center of the wafer holder. Moreover, the grinding disk is positioned in such a way that the rotation center of the semiconductor wafer reaches a working region or the edge region formed by teeth of the grinding disk. As a result, the entire surface of the semiconductor wafer can be ground without any movement in the grinding plane.
In the case of simultaneous double-side grinding (“double-disk grinding”, DDG), the semiconductor wafer is simultaneously processed on both sides in a manner floating freely between two grinding disks mounted on opposite colinear spindles, and in the process is guided in a manner substantially free of constraining forces axially between a water cushion (hydrostatic principle) or air cushion (aerostatic principle) acting on the front and rear sides, and is prevented from floating away radially loosely by a surrounding thin guide ring or by individual radial spokes.
During the grinding processes—this applies to both single-side and double-side grinding methods, it is necessary to cool the grinding tool and/or the processed semiconductor wafer. Water or deionized water is conventionally used as a coolant. Commercial grinding machines, such as e.g. the models DFG8540 and DFG8560 (“Grinder 800 Series”) from Disco Corp., which are suitable for grinding wafers having diameters of 100-200 mm and 200-300 mm, respectively, are equipped at the factory with a vacuum unit which ensures a constant coolant flow rate of 1 or 3 l/min (=liters per minute) during grinding, depending on the coolant temperature (constantly 1 l/min at a temperature of less than 22° C., constantly 3 l/min at a temperature of greater than 22° C.).
Double-side grinding machines are also available from Koyo Machine Industries Co., Ltd., for example. The model DXSG320 is suitable e.g. for the DDG grinding of wafers having a diameter of 300 mm. Both vertical and horizontal spindles are employed in combination with special diamond grinding tools. These grinding tools are designed such that they cut only with the edge and combine a rapid advance rate with little evolution of heat. The semiconductor wafer to be processed is fixed by hydrostatic pressure pads from both sides in a transport ring. The wafer is driven merely by means of a small lug which engages into the orientation notch of the semiconductor wafer. Stress-free mounting of the semiconductor wafer can be ensured in this way.
In the case of lapping, the semiconductor wafers are moved under a specific pressure with supply of a slurry containing abrasive materials between an upper and a lower working disk, which are usually composed of steel and normally provided with channels for better distribution of the lapping agent, whereby semiconductor material is removed. However, lapping is not part of the subject matter of the present invention.
DE 103 44 602 A1 and DE 10 2006 032 455 A1 disclose methods for the simultaneous grinding at the same time of both sides of a plurality of semiconductor wafers with a movement sequence similar to that of lapping, but characterized by the fact that abrasive is used which is fixedly bonded in working layers (“films”, “pads”) applied to the working disks. During processing, the semiconductor wafers are inserted into thin guide cages, so-called carriers, which have corresponding openings for receiving the semiconductor wafers. The carriers have an outer toothing which engages into a rolling apparatus comprising inner and outer toothed rings and are moved by means of said rolling apparatus in the working gap formed between upper and lower working disks. The carriers have openings through which coolant can be exchanged between lower and upper working disks, such that upper and lower working layers are always at the same temperature.
All the grinding methods mentioned leave behind significantly pronounced damage on the semiconductor wafer. Damage should be understood to mean crystal damage near the surface on account of the mechanical processing (“subsurface damage”). Scratches and other mechanically caused defects on the surface of the semiconductor wafer after grinding also constitute such damage. This crystal damage has to be eliminated by means of subsequent etching methods. However—as is known to a person skilled in the art—these etching methods adversely influence the geometry, in particular the edge geometry and the nanotopology, of the semiconductor wafer. On account of the poor nanotopology after etching, longer removal polishing processes are necessary in order to achieve the desired nanotopology.
Therefore, a person skilled in the art endeavors to minimize the damage after grinding and to be able to provide, after grinding, a semiconductor wafer having optimum geometry and nanotopology, but primarily with significantly less damage. This would allow the etching process possibly to be able to be dispensed with entirely. Primarily, however, a shorter process time could be made possible during polishing since, rather than correcting the nanotopology, obtaining an optimum surface roughness is to the fore.
DE 102007030958 claims a method for grinding semiconductor wafers, wherein the semiconductor wafers are processed in material-removing fashion on one side or on both sides by means of at least one grinding tool, with a coolant being supplied in each case into a contact region between semiconductor wafer and the at least one grinding tool, wherein a coolant flow rate is chosen in each case depending on a grinding tooth height of the at least one grinding tool and said coolant flow rate is reduced as the grinding tooth height decreases, whereby a constant cooling of the contact region between workpiece and grinding tool can be achieved by virtue of the fact that the coolant accumulates in front of the grinding teeth, flows around the latter and is swirled into the contact region between workpiece and grinding tool depending on the height of the grinding teeth. According to DE 102007030958, the amount of coolant which reaches said contact region is crucial for the grinding result (“subsurface damage”), and also the service life of the grinding tool.
What is disadvantageous about the method described in DE 102007030958 is the fact that the height of the grinding teeth has to be measured throughout the grinding process in order to be able to correspondingly adapt the coolant flow rate. This is because DE 102007030958 proceeds from a significantly increased coolant flow rate by comparison with the standard process, which coolant flow rate must also therefore be reduced as the tooth height decreases since an unchanged high coolant flow rate would otherwise unavoidably lead to aquaplaning effects.
For PPG, the method described in DE 102007030958 cannot be applied anyway since the grinding tools employed are not toothed grinding disks (“toothed wheels”) but rather working disks comprising working layers with abrasives bonded therein.